Three thoughts:
The biggest reasons to lock down hardware aren’t really there on the Deck. On top of that, it benefits Valve to have other devices running their storefront, so using off-the-shelf parts when possible makes it easier for others to use the Deck as a template.
That’s my thought as well – the licensed sports game market is probably more reliable (and generates more revenue vs. development cost) than originals, and they can “streamline” their business by splitting the originals business off for a sale. Not including sports probably makes the originals side more attractive to platform holders, as most sports games probably include provisions that require multiplatform releases (see also: the MLB signing a publishing deal for Sony’s “The Show” on non-Sony platforms).
We know why Dolphin wasn’t put on Steam.
Valve continues to host RetroArch and the various cores, so it’s not like they’re opposed to emulation in general. The ability to copyright “magic numbers” in the US (Valve is an American company) isn’t up for debate, and it would also put them in violation of the DMCA, so it’s not hard to see why Valve would be worried about this specific emulator.
As for Dolphin, they have options:
They had their moment in the early 00s, but they went away largely because people stopped buying them. Even though Hitz and Blitz showed some of the more-cringeworthy aspects of their sports, the sales were good enough for the licensors to not really care.
If Tape-to-Tape ends up selling well, you’ll see the NHL pushing for their own officially-licensed version (because nobody in that league has any original ideas). The same goes for other arcadey sports games.
MLB tried to bring back arcadey games with RBI Baseball (through their Advanced Media arm) – without it, the only annual baseball game would be Sony’s “The Show”. They wound that down when MLBAM signed a deal to publish The Show on non-Sony platforms. I’d think that, if the sales justified it, they’d be happy to continue selling both sim and arcade games.
Significant — you’d need to either get the old Linux build working (not an easy task today) or you can install it on Windows, copy the files over, and run it via Proton (but you’d need to manually add the registry key with your CD key to the Proton prefix’s registry).